The fundamental steps involved in the process of fabricating a semiconductor wafer are well-known in the prior art. Generally, the wafer begins as a blank substrate. Patterns are placed on layers formed on the wafer using well-known photolithographic techniques. Typically, the application of this pattern to the wafer is accomplished using photoresist technology. The photoresist is placed on the layer which is to be patterned and then exposed to light through a mask. The mask contains a "picture" of the image consisting of clear and opaque designs. These designs constitute the pattern to be created in the photoresist film and, through etching, onto the underlying layer.
When the photoresist is exposed to light the picture, or latent image, is created in the photoresist. The latent image is the replication of the photo intensity distribution in the exposed photoresist, constituting the pattern for the design that is to be etched into the underlying layer. The regions in the photoresist that are exposed to light are made either soluble or insoluble in a developer solution. In many cases, a post exposure bake step is required after exposure to create a solubility discrimination between the exposed and unexposed regions of the photoresist. If the exposed regions are soluble, a positive image of the mask is created on the substrate. These photoresists are referred to as positive resists. If the non-exposed areas dissolve in the developer, a negative image results. These photoresists are referred to as negative resists.
Photoresists are also characterized by the actinic (absorbing) wavelength used during exposure. Generally three categories exist. Resists exposed to ultraviolet (UV) with a wavelength between 360-700 nanometers (nm) are near-UV resists. Resists which are exposed to UV radiation having wavelengths between 300-360 nm are referred to as Mid-UV resists. Finally, resists which are subjected and photo-react to UV radiation with wavelengths of between 100-300 nm are referred to as Deep-UV resists. Using shorter wavelengths produces better resolution.
Furthermore, some resist systems utilize strong photoacids to catalyze a reaction to increase or decrease solubility of the exposed regions in positive resists or negative resists respectively. To facilitate the catalytic reaction, the wafer is post exposure baked (PEB) after exposure has created a latent image in it. However, a problem can arise where the time interval between exposure of the photoresist and baking of the resist is too long and the wafer sits in a clean room ambient. In this case, the acid used as a catalyst is neutralized. Thus, the acid at the surface of the photoresist will not be effective and the photoresist portion exposed to the atmosphere at the surface of the photoresist becomes insoluble. Hence, once the photoresist is subjected to developer, the developer solution diffuses beneath the insoluble portion of the resist and dissolves the soluble portion of the exposed resist. As a result, a cave-like pattern is created on the wafer and the image created as a result of the lithography process does not have a fine resolution.
The present invention overcomes this problem by avoiding the effects of the clean room ambient during the post exposure period. Furthermore, the present invention allows the time between exposure and baking to be increased, while protecting the image.